Simplex data communication systems, also known as one-way or passive systems, are employed in connection with paging. Generally speaking, one or more transmitters broadcast data communications. The communications include data which identify specific pagers. A population of pagers continually receive the broadcast communications. When one of the population of pagers identifies a communication directed to it, it alerts a subscriber to the incoming communication and often displays a numeric or alpha-numeric message carried by the communication. Due to the simplex nature of such systems, the system does not know when a pager fails to receive a communication directed to it. On the other hand, these simplex systems have many desirable features. Due to the absence of a transmitter and signal transmission capabilities in the pager, a small, low-power, light-weight, readily portable, and inexpensive unit results.
Conventional paging systems experience a problem related to a limited range. A paging system works only when its pagers reside within the area covered by the system's transmitters. When subscribers travel outside this area, their pagers cannot receive calls.
A related problem is that of limited paging capacity. As an area of coverage increases to better serve subscriber needs, the population of pagers likewise increases. As the population of pagers increases, the number of data communications increases. Thus, as the coverage area increases, a point of diminishing returns is reached. The number of data communications is so great that an unacceptable delay in the delivery of calls is experienced. Of course, system capacity could be increased by utilizing pagers which receive data communications over multiple channels. Unfortunately, this causes the cost of the pagers and paging service to increase to unacceptable levels.
Another problem with conventional paging systems is concerned with signal quality such as multipathing, interference, signal reflections, and the like. Generally, pagers are worn by users while engaging in their normal daily activities. These normal activities place the users inside automobiles, inside buildings, near large grounded structures, and in other spaces which electromagnetic signals have trouble penetrating. Consequently, the pagers' ability to receive communications varies considerably. To maximize a pager's ability to receive communications in all surroundings within the coverage area, paging systems are typically designed to utilize a low data-rate FSK modulation scheme in which transmitter power is boosted to a level that achieves an acceptable link margin. In addition, in simulcast paging systems the number of transmitters may be multiplied and geographically distributed throughout the covered area. The addition of transmitters and adjustment of power levels to achieve an acceptable link margin throughout the coverage area is typically a slow, trial-and-error process.
The use of satellites in paging addresses the limited range problem of conventional pager systems. However, the use of satellites presents its own problems. For example, satellites are typically constrained to using only low-power transmissions. Geostationary satellites are currently used to broadcast to terrestrial repeaters which can then transmit high power signals to nearby pagers. Proposals have been made to integrate a satellite with terrestrial transmitters in a simulcast paging system. However, this requires a satellite in a geostationary orbit, where the satellite is positioned a vast distance above the earth, where the satellite's transmission signal is extremely weak at the surface of the earth, and where the area of coverage on the earth's surface is exceptionally large.
Therefore, there is a substantial need for an improved message unit for use in a satellite-based, worldwide cellular messaging system.
There is also a substantial need for a message unit, in a multi-beam, satellite-based, world-wide cellular messaging system, that is capable of determining the optimum frames to monitor for messages.
There is also a substantial need for a message unit, in a satellite-based, world-wide cellular messaging system, that can synchronize very quickly to its message block.
There is also a substantial need for a message unit, in a satellite-based, world-wide cellular messaging system, that comprises a mechanism for achieving significant savings in its battery resource.